Update README.md

Author: Chuxel

containers/plantuml/README.md

@@ -10,15 +10,36 @@
 | *Definition type* | Dockerfile |
 | *Languages, platforms* | PlantUML |
 
-## Usage
+## Using this definition with an existing folder
 
-[See here for information on using this and other definitions in your project](../../README.md#using-a-definition). There are no addtional steps required.
+This definition does not require any special steps to use. Just follow these steps:
 
-### Try it
+1. If this is your first time using a development container, please follow the [getting started steps](https://aka.ms/vscode-remote/containers/getting-started) to set up your machine.
 
-1. Run **Remote-Container: Open Folder in Container...** in VS Code and select a cloned copy of this entire folder.
-2. The container will build and VS Code will connect.
-3. Open `sample.plantuml` in the `test-project` folder and hit Alt-D.
+2. To use VS Code's copy of this definition:
+   1. Start VS Code and open your project folder.
+   2. Press <kbd>F1</kbd> select and **Remote-Containers: Create Container Configuration File...** from the command palette.
+   3. Select the PlantUML definition.
+
+3. To use latest-and-greatest copy of this definition from the repository:
+   1. Clone this repository.
+   2. Copy the contents of `containers/plantuml/.devcontainer` to the root of your project folder.
+   3. Start VS Code and open your project folder.
+
+4. After following step 2 or 3, the contents of the `.devcontainer` folder in your project can be adapted to meet your needs.
+
+5. Finally, press <kbd>F1</kbd> and run **Remote-Containers: Reopen Folder in Container** to start using the definition.
+
+## Testing the definition
+
+This definition includes some test code that will help you verify it is working as expected on your system. Follow these steps:
+
+1. If this is your first time using a development container, please follow the [getting started steps](https://aka.ms/vscode-remote/containers/getting-started) to set up your machine.
+2. Clone this repository.
+3. Start VS Code, press <kbd>F1</kbd>, and select **Remote-Containers: Open Folder in Container...**
+4. Select the `containers/plantuml` folder.
+5. After the folder has opened in the container, open `sample.plantuml` in the `test-project` folder and hit <kbd>Alt</kbd>+<kbd>d</kbd> to see the illustration.
+6. Next, open `markdownsample.md` and see the embedded UML diagram.
 
 ## License